Piezoelectric transducer arrays of the type used for example for medical imaging, are normally formed with a plurality of substantially parallel piezoelectric elements, adjacent elements being spaced from each other by a predetermined distance. The space between the piezoelectric elements is typically filled with a substance chosen so as to minimize crosstalk and coupling between elements (i.e. spurious stimulation of one of the piezoelectric elements by an adjacent element), thereby minimizing the loss of both range and resolution caused by such effects.
Coupling and crosstalk between elements are a function of both the reflection coefficient between the piezoelectric element and the substance in the space between elements and the lossiness or absorption coefficient of the substance. To minimize these effects, the reflection coefficient should be as near to 1 as possible, and preferably at least 0.9. The absorption coefficient should also be relatively high. Since the reflection coefficient between two substances is equal to: ##EQU1## where d.sub.1 and d.sub.2 are the acoustic impedance of the propagating and receiving substances respectively, it is apparent that in order to minimize the reflection coefficient, the difference between the acoustic impedance of the substance in the space between the elements and the acoustic impedance of the piezoelectric elements should be maximized. Since the piezoelectric materials typically have a relatively high acoustic impedance, generally 25 to 30 megaRayls, although crystals with much lower acoustic impedance are available, while most gases such as air have a very low acoustic impedance, for example 1.03 meqaRayls for air, and air also has a high absorption coefficient, the space between the piezoelectric elements is typically left empty so as to be filled with air.
For purposes of the following discussion, two wavelengths will be defined. As is well known, the wavelength of a particular signal in a particular medium is equal to EQU .lambda.=.upsilon./f (2)
where
.lambda.=the wavelength of the signal in the medium. PA0 .upsilon.=the velocity of sound in the medium. PA0 f =the frequency of the signal. PA0 .lambda..sub..rho. =the piezoelectric wavelength. PA0 .upsilon..sub..rho. =the velocity of sound in the piezoelectric crystal medium. PA0 f.sub..mu. =the resonant or output frequency of the piezoelectric crystal. PA0 .upsilon..sub.o =the velocity of sound in the object to be scanned.
The first wavelength to be defined will be referred to as the "piezoelectric wavelength" (.lambda..sub.p). This wavelength is the wavelength of an acoustic signal in the piezoelectric element at the output frequency of the element or
Ti .lambda..sub..rho. =.upsilon..sub..rho. /f.sub..rho. (3)
where
The "object wavelength" (.lambda..sub.o) will be defined as the wavelength of a signal of frequency f.sub..mu. traveling at the velocity of sound in the object to be scanned by the transducer. Thus, EQU .lambda..sub.o =.upsilon..sub.o /f.sub..rho. (4)
where
It has been found that in order for the piezoelectric crystal to resonate in the normal operating environment for an ultrasonic transducer, the thickness of the piezoelectric crystal element should, for most piezoelectric substances, be substantially equal to one-half the piezoelectric wavelength (i.e. .lambda..sub..rho. /2). Further, in order to avoid grating lobes in the image obtained from the transducer, it is important that the periodicity or center-to center spacing between the piezoelectric elements be substantially equal to one half the object wavelength (i.e. .lambda..sub.o /2).
However, from equations 3 and 4 above, it is apparent that as the frequency of the piezoelectric element outputs increase, both the piezoelectric wavelength and the object wavelength decrease. Thus, at high frequencies, for example 10 MHz, the thickness of the piezoelectric element may be in the range of 100 to 200 microns (0.004" to 0.008") while the spacing between crystals required to achieve the desired periodicity may be in the range of 50 to 75 microns (0.002" to 0.003").
Heretofore, such piezoelectric transducer arrays have been formed by sawing or otherwise cutting a block of piezoelectric crystal which has a suitable backing bonded to it to form the desired spacing between piezoelectric elements. However, for high frequency applications where the spacing between piezoelectric elements is in the micron range, it is difficult, and sometimes impossible, to get saw blades which are thin enough, resulting in the thickness of the piezoelectric elements being less than optimum, and the spacing between elements being greater than is desired to avoid grating lobes.
Another potential problem with existing piezoelectric transducer arrays is that, since the space between the individual piezoelectric elements is filled only with air, structural support for the array is provided primarily by a backing layer. It is difficult to maintain accurate and uniform spacing between the elements in processing and use of the array without additional structural support. While various techniques such as cover layers have been provided for this purpose, such techniques have not always proved fully satisfactory, particularly in the processing of high frequency arrays having very small spaces.
A need therefore exists for improved methods of fabricating high frequency ultrasonic transducer arrays which permit the active piezoelectric transducer elements to be of desired width or thickness which permit optimum periodicity or spacing of active transducer elements, which permit the acoustic isolation between active piezoelectric elements to be maximized by having the space between the elements filled by a substance such as air providing the required impedance mismatch to achieve a high reflection coefficient, and which methods are relatively simple and inexpensive to perform. Preferably, the method will also provide enhanced structural support for the array at least during fabrication. A need also exists for various improved transducer arrays formed utilizing the above methods.